1. Field of the Invention
The present invention relates to superalloy compositions with a nickel base, for example intended for the manufacture of the disks of turbomachines, which may encounter temperatures up to 750.degree. C. so as to meet with the requirements of thermodynamic cycles of turbomachines of very high efficiency and specific power.
2. Summary of the Prior Art
Turbine disks must use materials of moderate density having a series of mechanical properties when hot such that:
1) Tensile properties up to 750.degree. C.: yield strength and elongation,
2) Creep strength up to 750.degree. C.: high strength and absence of notch sensitivity,
3) Resistance to low cycle fatigue; and
4) As low as possible of propagation of cracks taking into account environmental effects and the time for which they are subject to prolonged loading for which it is known that it will become critical in the temperature range concerned; the importance of this property is confirmed by the introduction of tolerance requirements to damage, as for example in the November 1984 Edition of the Standard MIL-STD-17-83 of the USAF.
The materials developed on powder metallurgy principals are currently most suitable for meeting the technical requirements and in the current state of the art there are used:
1) either materials which have a high resistance to crack-propagation with a low sensitivity to the environment but of which the yield strength and the resistance to creep are inadequate at high temperature. U.S. Pat. No. 3,147,155 describes examples of superalloy compositions of this type (see alloy A in Table 1 hereinabove).
2) or materials which have an elevated yield strength but of which the sensitivity to notching under creep conditions, the resistance to crack-propagation and the sensitivity to the environment are not satisfactory; U.S. Pat. No. 3,061,426 and FR-A-2 244 827 likewise describe examples of superalloy compositions of this type (see alloys R and I in Table 1 which are discussed hereinafter).
Several examples of these alloy compositions are given in Table 1.
The improvement of certain of the mechanical properties (resistance to crack-propagation for example) can be provided by achieving particular microstructures (coarse grains, necklace structure). These improvements are made however to the disadvantage of other characteristics (the yield strength for example) and the object of the present invention is to produce an optimum series of properties referred to hereinbefore by new alloy compositions.